specifically how do solutions of high or low ph change an enzyme's reactivity?

Answer 1

Enzymes are affected by changes in pH. The most favorable pH value - the point where the enzyme is most active - is known as the optimum pH. This is graphically illustrated here:

http://www.worthington-biochem.com/introBiochem/effectspH.html

Extremely high or low pH values generally result in complete loss of activity for most enzymes. pH is also a factor in the stability of enzymes. As with activity, for each enzyme there is also a region of pH optimal stability.

The optimum pH value will vary greatly from one enzyme to another, as Table II shows.

Answer 2

enzymes are proteins, and they're formed by amonoacids. aminoacids have a fixed structure, they always have an amino group (-NH2), and a carboxyl group (-COOH ), and a side chain that's specific to each aminoacid (sorry if i mess with the names, i learned it in portuguese). when forming a protein, the aminoacids' amino groups react with the acid group of the following aminoacid, and the side chain stays exposed. it is the interacting between the lateral groups that defines the shape of the protein, because the side chain may be basic, acid, neutral... and in different ph, the side chain may be positive or negatively charged. this makes some side chains attract others, and makes the protein fold itself in a way that's specific to each protein, and defined by the sequence of aminoacids that forms it.
now, concerning enzymes: the shape of an enxyme is very important to its work. the enzyme must match the shape of the molecules it's going to make react. so, each enzyme has a perfect shape, defined by the structure and interactions between it's aminoacids' side chains, and these interactions are influenced by the ph of the solution the enzyme is in.
changes in the ph change all the reactions, and so change the shape of the enzyme too, making it impossible to match the shape of the reacting molecules.